"Demon" carbs are no longer available. They were genuine imitation Holleys, anyway. An AED Holley will make more power than an equal sized Demon.

Rule of thumb, a 400 CID engine needs at LEAST a 750 CFM carb. With the 650, it will run out of "breath" right around 5,000. In olden times, the 800 CFM "spread bore" Holley (list # 6213) was the hot lick on a GTO.

What exactly ARE your goals for power? BTW, it was the Offenhauser Dual Port that kept your 400 from making good power. It's a door stop.